1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a photodiode for an image sensor by which dark current noise is reduced by providing a channel capable of discharging electrons accumulated on the surface of the photodiode.
2. Description of the Related Art
In general, an image sensor is a semiconductor device for converting optical signals into electric signals, and has a photo-sensing unit for sensing light and a logic circuit unit for processing the sensed light into electric signals to provide data. In a complementary metal-oxide semiconductor (CMOS) image sensor, MOS transistors are provided as many as the number of pixels using a CMOS technology, and a switching method is employed to sequentially detect output current using the transistors.
Typically, a unit pixel of an image sensor consists of a photodiode corresponding to the photo-sensing unit and four MOS transistors, which includes a transfer transistor used for delivering photo-charges generated from the photodiode to a floating node; a reset transistor used for discharging electrons stored in a floating diffusing area to detect signals; a drive transistor used as a source follower; and a selection transistor used for switching and addressing operations.
FIGS. 1A and 1B are a layout diagram and a cross-sectional view, respectively, illustrating a conventional N-type image sensor.
A P-type bulk semiconductor substrate (P+bulk) 21 having a high doping concentration is connected to a ground terminal GND, and a N-type impurity layer (PDN) 23 is formed inside a P-type semiconductor layer (P-sub) 22 having a low doping concentration. A P-type impurity layer (PDP) 24 having a high P-doping concentration is formed on the N-type impurity layer 23. In addition, a gate oxide film 27 is formed beneath a transfer gate 13, so that the transfer gate 13 is electrically insulated. The floating diffusion region (FD) 12 is connected to the photodiode (PD) 11 through the transfer gate 13. In addition, the floating diffusion region 12 is connected to a power voltage terminal (VDD) 15 through a reset gate (Rx) 14. In addition, a shallow trench isolation (STI) 25 and a channel stop layer (CST) 26 are formed to provide electrical isolation from an adjacent floating diffusion region 12 and connect the surface to the ground terminal GND.
FIG. 2 illustrates an equivalent circuit of the conventional N-type image sensor semiconductor of FIGS. 1A and 1B. Now, its operation will be described.
The reset transistor M1 is turned on when a voltage of the power voltage terminal 15 is supplied to the reset gate 14 of the reset transistor M1. In response, the voltage of the power voltage terminal 15 is delivered to the floating diffusion region 12 through the reset transistor M1, and the voltage of the floating diffusion region 12 is boosted.
Then, the transfer transistor M2 is turned on when the voltage of the power voltage terminal 15 is supplied to the transfer gate (Tx) 13 of the transfer transistor M2. In response, the boosted voltage of the floating diffusion region 12 is delivered to the cathode of the photodiode 11. As a result, the voltage is applied to the photodiode 11 in a reverse direction. Therefore, a depletion region of the photodiode 11 is enlarged.
When the voltages of the transfer gate 13 of the transfer transistor M2 and the reset gate 14 of the reset transistor M1 are switched to the voltage of the ground terminal GND, the transfer transistor M2 and the reset transistor M1 are turned off, so that the photodiode 11 is reversely biased.
FIG. 3 is a diagram for describing energy bands along a line B-B′ of FIG. 1B. When light is incident to the band, the light passing through the gate oxide film 27 creates an electron-hole pair inside a semiconductor. The light is diminished by the creation of the electron-hole pair, and light intensity and electron-hole pair generation density depending on a depth are also shown in FIG. 3.
Referring to FIG. 3, the electrons generated from the N-type impurity layer (PDN) 23 are directed to a center of the N-type impurity layer (PDN) 23, and holes are directed to the P-type impurity layer (PDP) 24 or the P-type bulk semiconductor substrate (P+bulk) 21. The holes directed to the P-type impurity layer (PDP) 24 are discharged via the channel stop layer 26, the bulk semiconductor substrate 21, and the ground terminal GND.
Specifically, the holes pass through the following path:
(1) P-type Impurity Layer 24→Channel Stop Layer 26→Bulk Semiconductor Substrate 21; or
(2) N-type Impurity Layer 23→Bulk Semiconductor Substrate 21.
On the contrary, the electrons may have a different path depending on where they come from. For example, the electrons generated from the N-type impurity layer (PDN) 23 are directed to a center of the N-type impurity layer (PDN) 23. However, the electrons generated from the surface of the photodiode 11 are accumulated on the surface of the photodiode 11 due to a surface band. When the amount of electrons accumulated on the surface of the photodiode 11 increases, the height of the surface band also increases, and the surface electrons flow to the N-type impurity layer (PDN) 23.
FIG. 4 is a timing chart of a signal supplied to the transfer gate 13 and the reset gate (Rx) 14.
Electrons may be generated from the surface of the photodiode 11 by thermal excitation as well as incident light. When the electrons generated by thermal excitation flow to the N-type impurity layer 23, they may produce dark noise, by which a signal may be erroneously generated even in a dark environment. This becomes a main factor of degrading dark characteristics of the photodiode.
Furthermore, in the conventional photodiode structure, the surface band serves as a barrier for blocking electric charges coming from the surface of the photodiode 11 and considerably affects characteristics of the photodiode. The surface band sensitively depends on a doping profile near the surface of the photodiode 11. Nevertheless, since the photodiode cannot appropriately discharge the electrons generated near the surface, it may be also sensitively affected by a surface potential sensitive to the doping profile of the surface. As a result, variations between pixels inevitably occur.
Such a phenomenon may similarly happen in a P-type photodiode. In the P-type photodiode, holes instead of electrons are accumulated on the surface.
As described above, in the conventional image sensor photodiode, when the electrons generated from the surface of the photodiode by thermal excitation flow to the N-type impurity layer, they may produce dark noise, by which a signal is erroneously generated even in a dark environment, and degrade dark characteristics of the photodiode.
Furthermore, since the photodiode cannot appropriately discharge the electric charges generated near the surface of the photodiode, it may be also sensitively affected by a surface potential sensitive to the doping profile of the surface. As a result, variations between pixels inevitably occur.